114 THE SIDE-CHAIN THEORY 



isms in twenty-four hours. If, then, constant quantities of the 

 bacterial emulsion (e. g., 1 c.c.) are treated with an increasing number 

 of agglutinin units it will be observed that the bacteria have the 

 power of absorbing an enormous excess of agglutinin beyond the 

 amount that is actually required to produce agglutination. If, 

 moreover, the number of units that is actually absorbed is compared 

 with the number added, the interesting fact develops that with 

 increasing concentration of the agglutinins the absolute absorption 

 by the bacteria rises, while the absorption coefficient, i. e., the ratio 

 between the number of units added and the amount absorbed, 

 falls. This is well shown in the accompanying table which is taken 

 from Eisenberg and Volk: 



ANTITYPHOID SERUM, ZOROASTER III. AGGLUTINATION VALUE = 45,000 UNITS 



Serum dilution. 



1 to 20000 . . . 



1 to 2000 . . . 



1 to 1000 . . . 



Ito 600 . . . 



1 to 500 . . .' 



1 to 200 . . . 



Ito 100 . . . 



1 to 20 . . .- 



1 to 4 .... 



1 to 2 ... 



1 to 1 ... 



The question then arises how to explain the apparent paradox 

 that the same quantity of bacteria which can only absorb 12,500 

 units out of 22,500 that have been offered can actually absorb 22,500 

 when brought in contact with a proportionately larger amount. 

 Upon first consideration the thought of a chemical union between 

 agglutinin and agglutinable substance would seem to be out of the 

 question. Various explanations, however, have been offered, any 

 one of which would show that the paradox is in reality only apparent. 

 As will be seen later on, there are reasons for supposing that an 

 agglutinating serum may contain not only one single agglutinin, but 

 a number of agglutinins which correspond to the presence of an 

 equal number of agglutinable substances (agglutinogens) in the body 

 of the bacillus; as experience, moreover, has shown that different 

 antigens, even though closely related, may differ very considerably 



